Chapter 24. Monitoring
Disk Usage

Each table has a primary heap disk file where most of the
data is stored. If the table has any columns with
potentially-wide values, there is also a TOAST file associated with the table, which
is used to store values too wide to fit comfortably in the main
table (see Section 49.2).
There will be one index on the TOAST table, if present. There may also be
indexes associated with the base table. Each table and index is
stored in a separate disk file — possibly more than one file,
if the file would exceed one gigabyte. Naming conventions for
these files are described in Section 49.1.

You can monitor disk space from three places: from
psql using VACUUM information, from psql using the tools in contrib/dbsize, and from the command line using
the tools in contrib/oid2name. Using
psql on a recently vacuumed or
analyzed database, you can issue queries to see the disk usage
of any table:

Each page is typically 8 kilobytes. (Remember, relpages is only updated by VACUUM, ANALYZE, and a
few DDL commands such as CREATE
INDEX.) The relfilenode value
is of interest if you want to examine the table's disk file
directly.

To show the space used by TOAST tables, use a query like the
following: